Many firearms, such as rifles, are equipped with optical sights, which use optics that provide the user with an image of an aligned aiming point or pattern (commonly known as a reticle) superimposed at the same focus as the target.
When shooting at long distances, shooters must adjust their aim to take into account the downward acceleration on the projectile imparted by gravity, which often referred to as “bullet drop.” This is typically done by adjusting the angular position of the riflescope relative to the rifle barrel using an elevation turret.
A zero point for a riflescope is determined when “sighting” a rifle at a known distance by adjusting the angular position of the riflescope relative to the rifle barrel, via the elevation turret, until the impact point of the bullet matches the point on the target coincident with the optical center of the riflescope reticle. For targets at greater distances than the distance used for establishing the riflescope's zero point, the elevation turret is used to adjust the angular position of the scope with respect to the rifle barrel to compensate for the greater amount of bullet drop.
The vast majority of hunting riflescopes have a single elevation zero point that is set to a single distance or elevation, e.g., 100 yards. Unless the riflescope's turret can be adjusted to match further distances beyond a single zero point, it is impossible to accurately and swiftly predict where a bullet will impact at middle to long distances without additional rapid adjustment aids.
Recently, riflescopes have been developed that include a turret with multiple indicators that each represent a zero point for various distances and scope elevation settings. Thus, a shooter can select an index indicator that corresponds to the distance of his target to adjust his riflescope to the proper elevation. One example of this type of riflescope is disclosed in U.S. patent application Ser. No. 12/068,098 to Menges et al. (hereinafter referred to as Menges). Menges discloses a riflescope turret with an inner coupling device surrounded by annular stacking indexing elements. Since the indexing elements stack on top of one another, the number of indexing elements that can be used is limited by their thickness with respect to the height of the coupler. As disclosed, a maximum of four indexing elements can be used, which limits resolution and accuracy potential. The number of available zero points or stops corresponds to the turret's elevation resolution; therefore, fewer zero points correspond to larger distances between zero points, which in turn results in a larger margin of error for distances between zero points. For example, if a shooter wanted to calibrate his riflescope for a range of 100 to 500 yards and had three available zero points, he could set the zero stops at 100, 300, and 500 yards, respectively. However, if five zero stops were available, he could set them at 100, 200, 300, 400, and 500 yards, respectively. In practice, for example, a target at 400 yards would be perfectly sighted for the system with five zero points, whereas the shooter with the three zero point system would have to set the turret at 300 yards and make manual adjustments to compensate for the remaining 100 yards.
A further limitation of modern riflescopes with multiple zero points, including Menges, is a limited rotational range of the turret, which affects turret range and/or resolution. The rotational range of a turret may be expressed in “minutes of angle” or MOA. Rotating the turret adjusts the angular position of the riflescope relative to the rifle barrel. The greater the target distance, the more MOA the turret must be rotated to compensate for the greater amount of bullet drop. The Menges turret has twelve MOA per 360° of rotation of the turret and the turret is limited to one turn, therefore limiting the range and/or resolution of the turret.
An even further limitation with modern riflescopes, including Menges, is the perceptibility of the indicators. Since each indicator zero point corresponds to a specific rotational angle of the turret, the width of the indicator zero point is limited by the arc length of the MOA resolution, and by the height of the indicator index. Riflescopes such as Mendes that use annular indicator indexes necessarily have very small indicator zero points, which are in the form of small colored dots, because the height of each annular index is limited by the overall turret height and the number of additional indices. Thus, it would be preferable to utilize index indicators that are as wide as the arc length of the turret's MOA resolution, and that are each as tall as the entire visible height of the turret.
An additional problem with current riflescopes is caused by the myriad distinctions between individual characteristics of ammunition, rifles, and atmospheric conditions. Ammunition and rifles each vary by brand and even by model within a given brand with respect to shot characteristics and manufacturing tolerances. Likewise, atmospheric conditions significantly vary depending on geographic location. For example, rifles used in northern Minnesota are subject to very different atmospheric conditions than those used in Afghanistan. In aggregate, there are countless possible combinations of parameters that have a direct effect on a given rifle's accuracy at various ranges.